Code Quality Rank: L5
Programming language: Swift
License: MIT License
Latest version: v2.2.0

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Build Status CocoaPods Version CocoaPods Plattforms Carthage compatible

The simplest Observable<T> implementation for Functional Reactive Programming you will ever find.

This library does not use the term FRP (Functional Reactive Programming) in the way it was defined by Conal Elliot, but as a paradigm that is both functional and reactive. Read more about the difference at Why I cannot say FRP but I just did.


  • [x] Lightweight, simple, cross plattform FRP
  • [x] Multithreading with GCD becomes a breeze
  • [x] Most of your methods will conform to the needed syntax anyway.
  • [x] Swift 3 and 4 compatibility
  • [x] Multithreading with GCD becomes a breeze via WarpDrive
  • [x] Supports Linux and swift build
  • [x] BYOR™-technology (Bring Your Own Result<T>)


  • iOS 7.0+ / Mac OS X 10.10+ / Ubuntu 14.10
  • Xcode 8


For a full guide on how this implementation works see the series of blog posts about Functional Reactive Programming in Swift or the talk at UIKonf 2015 How to use Functional Reactive Programming without Black Magic.

Creating and updating a signal

let text = Observable<String>()

text.subscribe { string in
  print("Hello \(string)")


Mapping and transforming observables

let text = Observable<String>()

let greeting = text.map { subject in
  return "Hello \(subject)"

greeting.subscribe { text in


Use functions as transforms

let text = Observable<String>()
let greet: (String)->String = { subject in
  return "Hello \(subject)"
  .subscribe { text in

Handle errors in sequences of functions

let text = Observable<String>()

func greetMaybe(subject: String) throws -> String {
  if subject.characters.count % 2 == 0 {
    return "Hello \(subject)"
  } else {
    throw NSError(domain: "Don't feel like greeting you.", code: 401, userInfo: nil)

  .then { text in
  .error { error in
    print("There was a greeting error")

This also works for asynchronous functions

let text = Observable<String>()
func greetMaybe(subject: String) -> Observable<Result<String>> {
  if subject.characters.count % 2 == 0 {
    return Observable(.success("Hello \(subject)"))
  } else {
    let error = NSError(domain: "Don't feel like greeting you.", code: 401, userInfo: nil)
    return Observable(.error(error))

  .then { text in
  .error { _ in
    print("There was a greeting error")

Flatmap is also available on observables

let baseCost = Observable<Int>()

let total = baseCost
  .flatMap { base in
    // Marks up the price
    return Observable(base * 2)
  .map { amount in
    // Adds sales tax
    return Double(amount) * 1.09

total.subscribe { total in
  print("Your total is: \(total)")

baseCost.update(10) // prints "Your total is: 21.8"
baseCost.update(122) // prints "Your total is: 265.96"


  • If you found a bug, open an issue.
  • If you have a feature request, open an issue.
  • If you want to contribute, open an issue or submit a pull request.


Dynamic frameworks on iOS require a minimum deployment target of iOS 8 or later. To use Interstellar with a project targeting iOS 7, you must include all Swift files directly in your project.


CocoaPods is a dependency manager for Cocoa projects. You can install it with the following command:

$ gem install cocoapods

To integrate Interstellar into your Xcode project using CocoaPods, specify it in your Podfile:

source 'https://github.com/CocoaPods/Specs.git'
platform :ios, '8.0'

pod 'Interstellar'

Then, run the following command:

$ pod install

swift build

Add Interstellar to your Package.swift:

import PackageDescription

let package = Package(
  name: "Your Awesome App",
  targets: [],
  dependencies: [
    .Package(url: "https://github.com/jensravens/interstellar.git", majorVersion: 2),


Carthage is a decentralized dependency manager that automates the process of adding frameworks to your Cocoa application.

You can install Carthage with Homebrew using the following command:

$ brew update
$ brew install carthage

To integrate Interstellar into your Xcode project using Carthage, specify it in your Cartfile:

github "JensRavens/Interstellar"


Why use Interstellar instead of [insert your favorite FRP framework here]?

Interstellar is meant to be lightweight. There are no UIKit bindings, no heavy constructs - just a simple Observable<T>. Therefore it's easy to understand and portable (there is no dependency except Foundation).

Also Interstellar is supporting BYOR (bring your own Result<T>). Due to its protocol based implementation you can use result types from other frameworks directly with Interstellar methods.


Interstellar is owned and maintained by Jens Ravens.


  • 1.1 added compability with Swift 2. Also renamed bind to flatMap to be consistent with Optional and Array.
  • 1.2 Thread was moved to a new project called WarpDrive
  • 1.3 WarpDrive has been merged into Interstellar. Also Interstellar is now divided into subspecs via cocoapods to make it easy to just select the needed components. The basic signal library is now "Interstellar/Core".
  • 1.4 Support swift build and the new Swift package manager, including support for Linux. Also removed deprecated bind methods.
  • 2 Introducing Observable<T>, the successor of Signal. Use the observable property on signals to migrate your code from Signal<T>. Also adding Linux support for Warpdrive and introduce BYOR™-technology (Bring Your Own Result<T>).
  • 2.1 Update to Swift 3.2 to make it compatible with Swift 4.
  • 2.2 Update to Swift 4.1, fixing build warnings on Xcode 9.3 (Maintains backwards compatibility with Swift 3.3 projects).


Interstellar is released under the MIT license. See LICENSE for details.

*Note that all licence references and agreements mentioned in the Interstellar README section above are relevant to that project's source code only.